In a vehicle in which an engine is mounted, as a transmission that appropriately transmits torque and rotational velocity generated by an engine to drive wheels according to the running state of the vehicle, an automatic transmission that automatically sets an optimal transmission ratio between the engine and the drive wheels is known.
Automatic transmissions mounted in a vehicle include, for example, multi-speed automatic transmissions that set a transmission ratio (a gear ratio) using a planetary gear apparatus and frictionally engaging elements such as a clutch and a brake, and belt-type continuously variable transmissions (CVTs) that continuously adjust a transmission ratio.
In a vehicle in which a multi-speed automatic transmission has been mounted, a transmission (gearshift) map that has gearshift lines (gear switching lines) for obtaining an optimal gear according to the vehicle speed and an accelerator opening degree (or throttle opening degree) is stored in an ECU (Electronic Control Unit) or the like. A target gear is calculated based on the vehicle speed and the accelerator opening degree with reference to the gearshift map, and based on that target gear, a clutch, a brake, a one-way clutch, and so on, which are frictionally engaging elements, are engaged or released in a predetermined state, and thus the gearshift level (gear) is automatically set.
In the configuration of a belt-type continuously variable transmission, a belt is wrapped around a primary pulley (input side pulley) and a secondary pulley (output side pulley) that are provided with a pulley groove (V groove), and by reducing the groove width of one pulley while increasing the groove width of the other pulley, the contact radius (effective diameter) of the belt to each of the pulleys is continuously changed to steplessly set a transmission ratio. The torque transmitted in this belt-type continuously variable transmission corresponds to the load that acts in the direction that the belt and the pulleys are made to contact each other. Accordingly, the belt is clamped by the pulleys such that tension is applied to the belt.
Also, transmissioning of the belt-type continuously variable transmission, as described above, is performed by enlarging or shrinking the groove width of the pulley grooves. Specifically, the primary pulley and the secondary pulley are respectively configured using a fixed sieve and a movable sieve, and a transmission is performed by moving the movable sieve frontward/rearward in the axial direction with a hydraulic actuator provided on the back face side of the movable sieve.
In this sort of belt-type continuously variable transmission, for example as disclosed in below Patent Document 1, the transmission ratio is controlled using an upshift transmission control valve and a downshift transmission control valve. A line pressure is supplied to these two transmission control valves as a source pressure.
A duty solenoid valve (below, referred to as a duty solenoid) is connected to the upshift transmission control valve and the downshift transmission control valve, and the upshift transmission control valve and the downshift transmission control valve are switched according to a control hydraulic pressure that is output by the duty solenoid. Thus, an oil amount supplied to the hydraulic actuator of the primary pulley via the upshift transmission control valve is controlled, and the amount of oil discharged from the hydraulic actuator of the primary pulley via the downshift transmission control valve is controlled. By controlling the hydraulic pressure of the hydraulic actuator of the primary pulley in this way, the groove width of the primary pulley, i.e., the belt contact radius of the primary pulley side, changes, and thus the transmission ratio is controlled.
Also, a belt clamping pressure control valve is connected to the hydraulic actuator of the secondary pulley. Line pressure is supplied to the belt clamping pressure control valve, and by supplying that line pressure to the hydraulic actuator of the secondary pulley by controlling the control hydraulic pressure output by the linear solenoid valve (below, also referred to as a belt clamping pressure control solenoid) as a pilot pressure, the belt clamping pressure is controlled.
The line pressure used for the above transmission control and belt clamping pressure control is produced by using a line pressure control valve (primary regulator valve) to adjust the hydraulic pressure generated by an oil pump. The line pressure control valve is configured to operate using a control hydraulic pressure that is output by a linear solenoid valve for line pressure control (below, also referred to as a line pressure control solenoid) as a pilot pressure.
As technology related to belt slippage of the belt-type continuously variable transmission, there is the technology disclosed in below Patent Document 2. In the technology disclosed in Patent Document 2, in a continuously variable transmission, slippage of the continuously variable transmission is determined based on the results of comparing an actually occurring transmission ratio state of change to a target transmission ratio state of change.
Also, in below Patent Document 3, technology is disclosed in which when occurrence of slippage between a rotating member (drive pulley) and a torque transmitting member (driven pulley) has been determined, in a controller that reduces the input torque of the continuously variable transmission, based on the state of slippage after the input torque has been reduced, the input torque is returned from the reduced state.    Patent Document 1: JP 2007-177833A    Patent Document 2: JP 2004-251359A    Patent Document 3: JP 2003-42276A